docs: fix comment

Author: MegaRedHand

src/lean_spec/spec/forks/lstar/validator_duties.py

@@ -92,7 +92,7 @@ def produce_attestation_data(self, store: LstarStore, slot: Slot) -> Attestation
         if justified_source.root == Bytes32.zero():
             justified_source = Checkpoint(root=store.head, slot=justified_source.slot)
 
-        # Sanity check: the source must be after target.
+        # Sanity check: the source must be before the target.
         assert justified_source.slot <= target_checkpoint.slot
 
         return self.attestation_data_class(